Agree 100% with Old Navy. Reasons such as: adjusting to new school/environment, difficulty understanding the prof, prof is a tough grader, time management issues, horrid roommate, too many pre-med students in the class, etc. will not resonate with USNA. Even the major issues that Old Navy mentions need to be really major. The reason is that most college/USNA students have to deal with various personal issues. If every issue affects your ability to do perform in the classroom, that doesn't bode well for you at USNA or in the fleet.
For example, I had a candidate actually apologize to me for not being able to attend CVW the upcoming WE. Why? Her mother had died of cancer earlier that week and the funeral was on the Saturday of CVW. I was amazed she'd come to the BGO interview and said as much -- she replied that her mother had been sick for a long time, that the interview was important to her (and her mother wanted her to do it) and that she hadn't wanted to inconvenience me! I'm not suggesting everyone would or should react this way . . . just that stuff happens in life and USNA and the USN/USMC expects that you'll handle most of that "stuff" in the normal course. Thus, if something happened in college or in h.s. to affect your grades, it really has to be a big deal in order for USNA to "consider" it.